Apparatus and methods for determining the minority carrier diffusion length (L) using the SPV method are well known. In brief, the principle of the diffusion length (L) determination requires the illumination of a specimen surface with monochromatic radiation of energy slightly greater than the bandgap of the semiconductor. Electron-hole pairs are produced and diffuse to the illuminated (front) surface where they are separated by the electric field of the depletion region (i.e., the surface-space-charge region) to produce a surface photovoltage (SPV). A portion of the SPV signal is coupled to an amplifier for amplification and measurement. The photon flux (photons per sq. cm. per second) is adjusted to produce the same magnitude of SPV at various wavelengths of illumination. The photon flux required to produce this constant magnitude SPV signal is conveniently plotted on the ordinate against the reciprocal of the absorption coefficient on the abscissa for each wavelength. The resultant plot is typically linear and is extrapolated to the zero photon flux intercept on the negative abscissa. This intercept value is the effective diffusion length (L). For a more detailed description of the theory and background for this method, see an article "A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors", by A. M. Goodman in the Journal of Applied Physics, Vol. 32, No. 23, pp. 2550-2552, Dec. 1961, and the article by A. M. Goodman entitled "Improvements In Method and Apparatus For Determining Minority Carrier Diffusion Length", International Electron Devices Meeting, Dec. 1980, pp 231-234. The American Society for Testing and Materials has adopted a standard using this method which is published as ASTM F 391-78. The ASTM standard, when implemented according to the block diagram of FIG. 1 of ASTM F 291-78, is provided particularly for testing the diffusion length (L) for minority carriers in silicon but the method in general may be used for other semiconductor materials.
See U.S. Pat. No. 4,333,051, incorporated herein by reference thereto, entitled "Method and Apparatus For Determining Minority Carrier Diffusion Length In Semiconductors", issued on June 1, 1982 to A. M. Goodman for a description of an apparatus using this method in which a servo system maintains a constant predetermined value of the SPV thereby allowing the measurements to be carried out in a relatively short time. The SPV pickup electrode described in this patent minimizes the effects of drift caused by laterally diffusing minority carriers during a test.
In the practice of the procedure for determining diffusion length by SPV it has sometimes been found that the plot of the photon flux I.sub.o) versus the reciprocal optical absorption coefficient (.alpha..sup.-1) is not a straight line as expected from conventional theory. The deviation has been determined to be caused by surface damage. Surface damage as used herein shall mean the damage caused by defects in the bulk region adjacent to and including the surface of a body of semiconductor crystalline material. This region shall be simply termed hereinafter as the bulk surface region of the body. Such defects can include crystallographic misorientations such as edge dislocations, screw dislocations, and twinnings. See C. Kittel, Introduction to Solid State Physics, Wiley & Sons, 1956, pp. 536-561, for a discussion of dislocations in crystalline bodies. Such damage is typically caused by the sawing, slicing and lapping operations performed in semiconductor processing. However defined, surface damage as used herein is to be distinguished from defects on the surface of the semiconductor material in the form of scratches, recesses, particulates, and the like.
It has been found that etching away a sufficient amount of the damaged crystallographic surface portion of a wafer results in a straight plot of the SPV measurements. However there is nothing in the art teaching how to make a determination as to whether there is crystallographic damage in the bulk surface region of a semiconductor body.